Traditional iron and steelmaking processes produce slag. Both ironmaking slag and steelmaking slag are considered to be waste products of these respective processes.
Steelmaking slag typically contains iron units in the form of FeO and flux units in the form of lime (CaO). Typically, the iron units are 35% by weight of the slag and the flux units are 25-35% by weight of the slag.
When the steelmaking slag is disposed of in accordance with prior art practice, the iron and lime units are lost.
Significant economic and environmental benefits would be gained if it were possible to recover or reuse iron and lime units from steelmaking slag.
Recycling of steelmaking slag to prior art ironmaking vessels such as traditional blast furnaces in order to recover or reuse the iron and lime units is not considered to be a viable option. One reason for this is that phosphorus in feed materials that are supplied to typical blast furnaces tends to partition to the molten iron produced in the furnace. When this molten iron is subsequently supplied to a steelmaking vessel, the phosphorus partitions into the steelmaking slag in order to produce steel of required chemistry. Thus, recycling steelmaking slag to a typical blast furnace is not practical and would simply result in an increasing phosphorus load in down stream steelmaking vessels. This is undesirable.
Whilst blast furnaces are the traditional option for producing iron from iron ore, various molten bath-based direct smelting processes have been proposed with a view to avoiding disadvantages of blast furnaces, such as the need for sintering and coke making operations. Such direct smelting processes include, by way of example, the Romelt, DIOS (Direct Iron Ore Smelting), and HIsmelt processes.
The Romelt process operates at ambient pressure using a molten iron and slag bath with ore fines and non-coking coal dropped in from above. Oxygen and an air/oxygen mixture are injected at each of two elevations through side tuyeres to stir the melt and post-combust bath gas.
The DIOS process is another molten iron and slag bath-based process and operates under pressure (1 to 2 bar g) and uses top-feed of coal and ore. Unlike Romelt, it uses a top lance for oxygen injection and has a fluidised bed system for iron ore pre-reduction. In the smelter, most of the reduction occurs in a foamy slag layer. There are significant temperature and FeOx gradients within the reactor (higher temperatures and higher FeOx levels at the top of the slag layer).
The HIsmelt process developed by the applicant is another molten iron and slag bath-based process and differs from ‘deep slag’ smelters such as DIOS and Romelt in that the solids fed to the vessel are injected significantly deeper into the melt. This leads to stronger mixing in the vessel, with hardly any temperature gradients in the liquid. The HIsmelt process is described, by way of example, in International applications PCT/AU96/00197 (WO 96/31627) and PCT/AU01/00222 (WO 01/64960) in the name of the applicant and the disclosure in these International applications is incorporated herein by reference.
It has previously been reported that the HIsmelt process is able to process iron ores such as high phosphorus Brockman ores and produce a pig iron with less than 0.05% phosphorus for downstream steelmaking. In contrast to the traditional blast furnace, the HIsmelt process has a more oxidising slag which results in extremely effective partitioning of phosphorus to the slag. During pilot plant testing it was reported by the applicant that 90 to 95% of the phosphorus fed to the HIsmelt pilot plant vessel reported to the slag. The DIOS and Romelt processes are also believed to preferentially partition phosphorus to slag.
It has also previously been reported that steelmaking slag in the form of fines can be used as a feed material that is supplied directly to a direct smelting vessel operating in accordance with the HIsmelt process—see paper entitled “HIsmelt—Competitive Hot Metal from Ore Fines and Steel Plant Wastes” by the subject inventor, R J Dry, and others, prepared for METEC Congress, 14-15 Jun. 1999. The left hand column on page 4 of the paper mentions that steel plant reverts, including BOF slag, can provide carbon credits that reduce the coal requirements for the HIsmelt process flowsheet shown in FIG. 3 of the paper.